CHAP. XVI 



THE BRITISH ISLES 



355 



The acknowledged increasing abundance specifically and 

 individually of the more primitive species of our land and 

 freshwater shells, as we proceed westward and northward 

 in the British Isles, points to the gradually lessening com- 

 petition to which they are subjected by the stronger and 

 more highly organised forms, more especially characteristic 

 of Eastern England. 



These more dominant species are gradually dispossess- 

 ing the weaker races of districts in which they formerly 

 existed, leading to their more decided restriction to 

 western areas, and to a more marked discontinuity of the 

 localities some of them still inhabit in Eastern England. 



The affinity shown by the flora of South-west Ireland 

 to that of South-western Europe, is likewise to a certain 

 extent shared by the MoUusca : the fact is of great interest, 

 and displays the last footholds of these waning species in 

 Western Europe. 



The North American species or varieties of land and 

 freshwater shells discovered in the West of England and 

 chiefly in the Lancashire district, where so many able and 

 enthusiastic conchologists are congregated, may probably 

 be artificial through unintentional introductions due to the 

 importation of cotton and other products from the United 

 States, more especially during the progress of the American 

 Civil War ; but it must not be overlooked that the geo- 

 graphical position occupied by these forms is in broad 

 harmony with the western habitat in our islands of those 

 American species of plants and other organisms which still 

 linger in this country, and leads us to speculate upon the 

 possibility of these species being also relics of a former 

 greater abundance and wider distribution in this country 

 of these and probably other forms, which are now restricted 

 to the American continent or other remote regions. 



The appended list of British species and varieties of 

 land and freshwater shells contains no forms recorded for 

 continental Europe, yet in all probability this apparent 

 difference is really due in many cases merel}^ to the 

 particular forms not having been, as yet, separated or 

 distinguished, or because they are differentiated under 

 other names, and thus to some extent secured from easy 



